The present invention relates to an amplifying solid-state imaging device and a method for driving the same.
In recent years, demand for a device used for sensing the one- or two-dimensional distribution of light quantity has tremendously increased. In the field of solid-state imaging devices, a so-called xe2x80x9camplifying solid-state imaging devicexe2x80x9d has been an object of vigorous research and development these days. Such an amplifying solid-state imaging device includes a plurality of pixels, each of which includes a photoelectric transducing section, a storage section and a sensing circuit. The photoelectric transducing section receives incoming light and photoelectrically converts the energy of the light into electrical energy to create signal charge. The storage section stores the signal charge thereon. The sensing circuit includes an amplifying transistor for outputting a signal in accordance with the quantity of the signal charge. The storage section is connected to a control terminal section of the amplifying transistor (e.g., gate electrode of an MOS transistor, base of a bipolar transistor, etc). An output value of the sensing circuit is controlled using a potential at the storage section, which is variable with the quantity of signal charge.
Such an amplifying solid-state imaging device includes a plurality of amplifying transistors, each functioning as a sensing circuit, for the same number of pixels. Even if these amplifying transistors are formed within a single device or on an identical substrate by the same process, the characteristics of these transistors are not totally uniform. For example, if the threshold voltages Vt are variable among these transistors functioning as sensing circuits, then output values thereof are also variable, even though quantities of incoming light received by respective photoelectric transducing sections and resulting potentials at respective control terminal sections are equal to each other. As a result, spatially fixed pattern noise (FPN) is created, which considerably deteriorates the resultant image quality.
An object of the present invention is providing (1) an amplifying solid-state imaging device that can read out information from a storage section more accurately and rapidly by compensating for the effects produced by a variation in characteristics of amplifying transistors as sensing circuits for respective pixels irrespective of the quantity of light received and (2) a method for driving the device.
To achieve this object, the present invention provides unit compensators, each including: first and second storage devices implementable as MOS capacitors; and a switching device for electrically connecting or disconnecting these storage devices to/from each other, for respective columns of pixels.
Specifically, an amplifying solid-state imaging device according to the present invention includes: a photoelectric transducing section changing from a first potential state corresponding to a reset operation into a second potential state variable with an intensity of incident light or vice versa; an amplifier for sensing the first and second potential states of the photoelectric transducing section, thereby outputting first and second signals, respectively; and a compensator for receiving the first and second signals from the amplifier and outputting a third signal. The compensator includes: a first storage device implemented as an MOS capacitor with first and second electrodes; a second storage device implemented as another MOS capacitor with first and second electrodes; means for applying a fixed potential to the first electrode of the second storage device; a switching device for electrically connecting or disconnecting the second electrodes of the first and second storage devices to/from each other; means for applying a signal potential, corresponding to the second signal, to the first electrode of the first storage device; means for supplying charge to the respective second electrodes of the first and second storage devices such that the same reference potential is applied to the second electrodes of the first and second storage devices; means for applying a reset potential, corresponding to the first signal, to the first electrode of the first storage device instead of the signal potential; means for turning the switching device ON such that while the reset and fixed potentials are applied to the first electrodes of the first and second storage devices, respectively, charge is transferred between the respective second electrodes of the first and second storage devices to equalize potentials at the respective second electrodes of the first and second storage devices with each other; and means for outputting the third signal, corresponding to a quantity of charge stored on the second storage device, with the switching device turned OFF after the charge has been transferred.
In one embodiment of the present invention, the switching device is implementable as an MOS transistor with a gate electrode, and the gate electrode of the switching device preferably overlaps partially with the respective first electrodes of the first and second storage devices. In this particular embodiment, the gate electrode of the switching device and the respective first electrodes of the first and second storage devices are preferably formed out of respective polysilicon films deposited over a silicon substrate with an insulating film interposed therebetween.
In another embodiment, the charge supply means may include means for supplying the charge to the second electrode of the second storage device through the second electrode of the first storage device while the switching device is turned ON. Alternatively, the charge supply means may include means for supplying the charge to the second electrode of the first storage device through the second electrode of the second storage device while the switching device is turned ON.
In still another embodiment, the amplifier may be an amplifying transistor, the current drivability of which is variable with the potential state at the photoelectric transducing section. The imaging device may further include a load device for generating potential signals, corresponding to a current flowing through the amplifying transistor, as the first and second signals. In still another embodiment, the first electrodes of the first and second storage devices are preferably formed by a different process from an electrode of the amplifier.